1,047 research outputs found
Stress Concentration in Nonlinear Creep of a Simple Shell
No full textA long, thin circular cylindrical shell is loaded at one edge by symmetrical radial shear Qx and bending moment Mx. (No interior pressure.) The shell is made of material which under applied stress creeps with a strain rate which is proportional to the rth power of the stress. Previous results are used to derive, approximately, the greatest stress in the shell for any Qx, Mx, and r. It is shown that for any load the greatest stress decreases as r increases, and is approximately a linear function of 1/r. The case r = 1 is exactly analogous to a linear elastic problem, and the case r → = ∞ corresponds exactly to a perfectly plastic problem. Results for any exponent r may thus be found approximately by simple interpolation between results obtained in linearelastic analysis and perfectly plastic analysis.</jats:p
Shell buckling, without ‘imperfections’
No full text The buckling behaviour of thin shell structures under load has been a persistent challenge to engineering designers and researchers over many decades. In this article I consider two unusual experimental studies on the buckling of thin-walled elastic cylindrical shells, each of which sheds intriguing light on the buckling phenomena. The classical theory of buckling of thin cylindrical shells under axial compression predicts that the buckling stress will be proportional to t/ R– the ratio of thickness to radius – other things being equal. But collected results of experimental studies from many laboratories, when plotted on log–log scales, show clearly that the buckling stress is actually proportional to ( t/ R)1.5, with the measured buckling stresses being scattered through a factor of about 4 for shells with R/ t > 200. Such scatter is commonly judged to be in accord with Koiter’s theory of ‘imperfection sensitivity’. But that theory lays no claim to an understanding of the empirical 1.5-power law. I claim that a key to this situation is the experimental performance of some small-scale open-topped silicone rubber shells, buckling under their own weight, which clearly demonstrates a 1.5-power law, but with very little scatter. The buckling mode of these shells involves almost entirely inextensional deformation, with a single small dimple growing near the base, separated from the rest of the shell by a narrow boundary layer that accounts for almost all of the dimple’s elastic strain energy. A straightforward, simple analysis of the mechanics of the dimple is consistent with the experimental 1.5-power law. As noted above, experimental buckling loads of shells that are closed at both ends also show the empirical 1.5-power law, but now with significant statistical scatter. A second aim of the paper is to throw light on that phenomenon. I venture to attribute it to the effect of the boundary conditions of the shell. I adduce support for this view from experimental observations on the buckling of a shell with special, frictional end-fittings. That feature produces significantly higher collapse loads, and with much smaller scatter, than for corresponding shells with fixed boundaries; and it permits striking pre-buckled deformations to occur, of a kind not previously noted. It will be appreciated that neither of the two parts of this article depends on the widely accepted theory of imperfection-sensitivity; hence my choice of title. It is a pleasure for me to submit this article to a special publication in honour of Michael Rotter, with whom I have discussed matters of this sort through three decades. </jats:p
Impacts of birds of prey on gamebirds in the UK: a review
No full textThe influence of predators on the distribution, density and dynamics of their prey species has long been of interest to ecologists and wildlife managers. Where the prey population is also utilised by humans, conflicts may arise through competition for a limited resource. Because gamebird shooting in the UK provides employment, recreation and income, the impact of birds of prey on gamebird populations has been the subject of intense debate for many years. A variety of approaches has been used to assess the impacts that raptors have on gamebird populations. Here we review the applicability and limitations of the methods used and assess the scientific evidence for population-level and economic impacts of raptors on gamebird populations in the UK. Raptors may, in some situations, take large numbers of gamebirds and may be an important proximate cause of mortality, although few studies have assressed the impacts of raptors on either breeding or pre-shooting densities. Two exceptions are studies of Hen Harrier and Peregrine predation on Red Grouse on moorland in Scotland and Sparrowhawk predation on Grey Partridge on farmland in England. Both these studies suggested that raptors could have population-level impacts when their gamebird prey was already at low density. Studies on predation of captively bred gamebirds suggest that numbers taken by raptors at release pens vary considerably and in a few cases raptors have been documented killing relatively large numbers. On the whole, however, it appears that raptors account for a relatively small proportion of mortality among released birds and the impact on subsequent shooting bags is unknown. We summarise important gaps in current knowledge and recommend specific areas for future research
Folding the Carpenter's Tape: Boundary Layer Effects
No full textThe "carpenter's measuring tape"is a thin spring-steel strip, preformed to a curved cross section of radius R, which is straight when being used for measuring. Under bending moments, it forms a localized hinge, in which the transverse curvature is suppressed, and the longitudinal radius r is approximately equal to R. Rimrott made a simple strain energy analysis of the hinge region for isotropic material, which predicted that r = R. Both experimental observations and finite element computations show that ζ = r/R > 1, where the value of ζ exceeds unity by up to 15%, depending on whether the tape is bent in "equal-sense"or "opposite-sense"curvature; ζ varies linearly with Poisson's ratio in both cases. We make a minor change to Rimrott's analysis by introducing a boundary layer, in order better to satisfy the physical conditions at the free edges; this successfully accounts for the observed behavior of the tape
IUTAM-IASS Symposium on Deployable Structures: Theory and Applications Proceedings of the IUTAM Symposium held in Cambridge, U.K., 6–9 September 1998
No full textThe IUTAM-IASS Symposium on Deployable Structures: Theory and Applications was held on 6-9 September 1998 in the Department of Engineering, University of Cambridge. The Scientific Committee was appointed jointly by the two sponsoring organisations, the International Union for Theoretical and Applied Mechanics (IUTAM) and the International Association for Shell and Spatial Structures (lASS). Its membership was as follows: S. Pellegrino (UK, Chairman) C. R. Calladine (UK) L. B. Freund (USA) M. Geradin (Belgium) Y. Hangai (Japan) K. Kawata (Japan) B. Kroplin (Germany) S. ]. Medwadowski (USA) M. M. Mikulas (USA) A. Samartin Quiroga (Spain) F. P. ]. Rimrott (Canada) T. Tarnai (Hungary) The opening lecture of the Symposium was given by Professor C. R. Calladine. It was followed by seven lecture sessions on Concepts, Structural Mechanics, Reflectors, Deployable Structures in Nature, Design and Testing, Inflatable Structures, Simulation, plus a large interactive session during which posters and physical models were shown. A discussion of topics suggested by the participants took place during the closing session. There were 83 participants from 18 countries, and a total of 50 papers were presented. Financial support was generously provided by the International Union of Theoretical and Applied Mechanics (IUTAM). Additional support was provided by the British Council, the British National Space Centre (BNSC), and Kluwer Academic Publishers. We thank the members of the Scientific Committee for their willingness to referee to a high standard both the abstracts and the papers for the proceedings. Professor G. M. L
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Collapse of masonry structures
No full textThis dissertation examines the collapse of masonry structures in response to large support displacements and horizontal ground accelerations. There are two main classes of masonry structure: arches that thrust, and supporting elements, such as walls and buttresses, which resist the thrust. This dissertation analyses the safety of arches and buttresses and identifies the resulting collapse mechanisms due to support displacements or horizontal accelerations. In particular, this resesarch investigates the stability of a masonry arch supported on buttresses and the conditions necessary for collapse to occur. Engineers are frequently asked to determine the safety of masonry
structures that have been severely distorted over the years, often due to subsidence or other long-term movements in the foundations, and this dissertation provides guidance in the assessment of such structures. The resistance of masonry buttresses to high-level horizontal loads is examined. In the case of failure due to overturning, a fracture will develop in the masonry, significantly reducing the resistance of the buttress. The capacity is further reduced by outward leaning of the buttresses, a common source of distress for masonry structures due to movements in the supporting foundations. Based on these considerations, new measures of safety are proposed for buttresses under horizontal loading. Outward leaning of the buttresses increases the span of the arch or vault. Spreading supports will cause large deformations in the arch, which increase the horizontal thrust of the arch and may lead to collapse. In addition, lateral ground accelerations can cause the collapse of arches. The influence of seismic action can be approximated to first order by equivalent static analysis to determine the initial
collapse mechanism. These problems are analysed for circular masonry arches, and the collapse conditions are identified for various geometries. The findings are combined to investigate the stability of the masonry arch supported on buttresses. The safety of the system is examined by studying the influence of imposed displacements. As the buttresses lean, the thrust of the vault increases and the resistance of the buttress decreases. The collapse mechanisms are identified for both the static case of leaning buttresses and the dynamic case of horizontal acceleration. This analysis illustrates that the arch will collapse and the buttresses will remain standing in most cases. Based on these considerations, new methods are proposed for assessing the safety of masonry structures and determining the influence of future movements on the stability of existing masonry structures
Assignment of phosphate groups and determination of P,H coupling constants in the DNA fragment d(CGTACG) by31P 2D NMR experiments
No full text2D 31P/1H correlation spectroscopy has been used to assign individual phosphates in the self-complementary DNA fragment d(CGTACG), which is present in solution as a B-DNA type double helix. 31P chemical shift variation with temperature and with the base sequence has been studied, and evidence is given of a steric clash between purines in the minor groove, affecting the chemical shift of the innermost TpA phosphate. For this hexanucleotide there is a good correlation of 31P shifts with both the helical twist (∑1) and roll angle (∑2) Calladine-Dickerson sum functions. Different types of linear correlations, also including data from other nuclotides, have been performed. The results show that it is more appropriate to consider the change in chemical shifts as it occurs during melting rather than the actual 31P shifts. An interesting correlation was also found between 31P chemical shifts and P,H-3′ coupling constants
The theory of thin shell structures 1888{1988
No full textwhere D u is the flexural rigidity of the strip in the longitudinal direction, D 22 is the flexural rigidity in the transverse direction, and D 12 is the cross-term. Then using (4) we have + ¥ D^D * Multiplying by the total area of region (a); i.e., b^r, we obtain a simple expression for the total strain energy, U T , in this region: The key observation of Rimrott was that we may ignore the transition region in seeking a solution to the present problem. This is so since the transition region is a more or less constant feature of the problem in the sense that it is independent of r. Additionally, the length of the transition region is observed to be approximately equal to the width of the tape, and this is small compared to the length of the region (a) which is \j/r. Hence, we expect the amount of elastic energy stored in this region to be small compared to that stored in region (a). We may also neglect the elastic energy stored in the nearly straight regions. Then the total energy in the system, which varies with r, is given in Eq. An Experiment We now describe an interesting and extremely simple experiment to which Eq. (2) can be applied. Machine-made paper is an anisotropic (orthotropic) elastic material. Typically, for instance, photocopy paper has an elastic modulus in one (the "machine") direction about two times that in the other ("cross") direction. We take two sheets of such paper, and lay one over the other with one sheet turned 90 degrees with respect to the second. We then roll the two sheets together into a tube of diameter approximately 30 mm, and place a band around the tube so that it will not open. Now place the rolled tube into an oven at about 125 °C for approximately ten minutes. Remove the tube from the oven and permit it to cool. When the band is removed the tube will very nearly maintain its restrained diameter due to viscoelastic relaxation of the natural polymers at the elevated temperature. Now cut a strip of width about 10 mm from each of the sheets in the rolled tube along the longitudinal direction of the tube. Perform the experiment indicated in where r MD and r C D are the radii which will be formed, respectively, for the strip cut in the machine direction and the crossdirection of the paper. If for each of the two strips we maintain the straight sections of the strips parallel we readily perceive the 2:1 ratio
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